The present invention relates to a radioactive ray detecting device used as a radioactive ray detector to measure the energy spectrum of a radioactive ray including an electromagnetic wave and an electron beam, such as an X ray and .gamma. ray, and a corpuscular ray, such as an .alpha. ray.
A conventional radioactive ray detecting device using a superconductor tunnel junction is structured by a very thin oxide film (not shown) of approximately 1 to 2 nm formed by oxidizing a surface of a conventional first superconductor thin film 8 formed on a substrate 7 as in FIG. 3, and a conventional second superconductor thin film 9 further provided thereon.
A silicon single crystal or sapphire single crystal with an oxide film is used as a material of the substrate 7. Also, a sputter film or evaporation film such as tin, lead, niobium or tantalum is used as the conventional first superconductor thin film 8. A sputter film or evaporation film such as tin, lead, niobium or tantalum is also used for the conventional second superconductor thin film 9.
The radioactive ray incident inside the conventional first superconductor thin film 8 causes quasiparticles at the inside. The quasiparticles in this case can be considered to be usually electrons, which move due to diffusion. If they move to the vicinity of the oxide film within their lifetime, they pass through the oxide film by a tunnel effect and are collected as signal charges by the conventional second superconductor thin film 9.
Because the amount of signal charges is proportional to an incident radioactive ray energy, it is possible to measure a radioactive ray energy.
In the conventional structure, the conventional superconductor thin film 8 is formed on the substrate 7. The quasiparticles caused in the first superconductor thin film 8 due to the radioactive ray pass through the oxide film by the tunnel effect and are collected as signal charges by the conventional second superconductor thin film 9. However, the loss of quasiparticles occurs at an interface of the conventional first superconductor thin film 8 and the substrate 7, reducing the signal charge collection efficiency.
Also, although part of the energy of the radioactive ray causes phonon in the conventional first superconductor thin film 8, a phonon possessing a sufficient energy further causes quasiparticles thus contributing to signal charges. This phonon also causes a loss at the interface, if the substrate 7 is present.
Furthermore, because the conventional first superconductor thin film 8 formed on the substrate 7 is polycrystalline, the quasiparticles causes a loss at grain boundaries, reducing the signal charge collection efficiency.
If the charge collection efficiency is low, the variation in the statistically measured charge amount increases so that the energy measurement value is increased in variation thus worsening energy resolving power.